Silicon base ceramics exhibit accelerated oxidation rates in high temperature, aqueous environments such as for example, the combustor and turbine sections of gas turbine engines. In order to reduce the rate of oxidation on silicon base substrates used as ceramic components in such environments, significant effort has been given to providing environment barrier coating, i.e., barrier layer(s), for the silicon base substrates so as to increase the service life of such component parts.
With reference to FIGS. 1a and 1b, prior art environmental barrier coatings form a composite 10 comprising a silicon base substrate 12, a bond coat or layer 14 such as a dense continuous layer of silicon metal, a barrier layer 16 such as either an alkaline earth aluminosilicate base on barium and strontium, or yttrium silicate, and an optional top layer such as a refractory oxide and/or silicate layer 18 or other metal oxide such as for example zirconium oxide. In addition, all intermediate layer 20 may be provided between the 14 bond coat and the barrier 16 and/or between the barrier layer 16 and the top layer 18. The intermediate layer comprises, for example, a mixture of the barrier layer material with an additional oxide such as mullite. These prior art environmental barrier systems have proved for many silicon base substrates to be protective with respect to oxidation of the silicon base substrate and, in addition, are adherent. However, it has now been found that certain mechanical properties of some silicon containing substrates especially those substrates having CTE's less than silicon and those which are brittle, such as silicon nitride, suffer a significant reduction in 4-point bend strength at room temperature.
Naturally, it would be highly desirable to provide environmental barrier coatings for silicon containing substrates such as silicon nitride which do not result in significant loss of mechanical properties.
Accordingly, this is a principal object of the present invention to provide bond coats for silicon base, molybdenum base and niobium base substrates which does not have significant adverse affect with respect to mechanical properties.